Calcium phosphates are known as physiologically acceptable biomaterials potentially useful as hard tissue prosthetics. The most widely studied of these are hydroxyapatite and tricalcium phosphate. When these materials are shaped and made porous they can be used alone or as a supplement or extender with bone for hard tissue prosthetics. Under appropriate conditions and with an appropriate form of calcium phosphate, the calcium phosphate is resorbed and new bone growth results. Porositizing the calcium phosphate results in increased surface area so that more calcium phosphate will be exposed to the body fluids facilitating resorption and growth of new bone tissue. Excellent control of the type and size of pores formed can be achieved by compaction of calcium phosphate powders containing naphthalene followed by removal of the naphthalene by leaching or sublimation. Hydrothermal exchange of marine coral structures (i.e., calcium carbonate for calcium phosphate), and decomposition of hydrogen peroxide have also been employed to generate pore filled structures.
The dense or "green" forms of the calcium phosphate implant materials have mechanical properties equal to or exceeding that of natural bone, but their respective porous forms do not, thus severely limiting their usefulness as hard tissue prosthetics.
It is known that certain natural and synthetic polymers can be used alone or in conjunction with other materials for bone prostheses or other implantable devices. Natural polymers include collagen (U.S. Pat. No. 4,192,021) and gelatin (German Patent No. 2,812,696). Synthetic polymers include polyacrylates, poly(methylmethacrylate), polyethylene, polysulfones, polyamides, polyesters, polytetrafluoroethylene, and polycarbonates (Great Britain Patent Application No. 2,031,450A); polyacetates and polyglycolates (U.S. Pat. No. 4,192,021); epoxides, polyacrylamide, polypropylene, polyurethanes, polyacetals, silicone resins, and furan resins (U.S. Pat. No. 4,222,128); polyvinyl pyrrolidone, polyvinyl alcohol (U.S. Pat. No. 4,263,185); and a cross-linked pentapeptide (U.S. Pat. No. 4,187,852). The natural polymers and some of the synthetic polymers are resorbable, i.e., biodegradable.
The art also teaches that nontoxic water soluble substances such as sodium chloride can be incorporated into a mixture of powdered acrylic polymer, liquid monomer, and other ingredients in a mold and the mixture polymerized to produce a shaped composite. The composite can then be made porous by leaching the sodium chloride with water (U.S. Pat. No. 4,199,864).
The various polymer-calcium phosphate composites are prepared in a number of ways including blending calcium phosphates with polymeric binder and subsequent molding (Great Britain Patent No. 1,593,288); impregnation of sintered, porous calcium phosphate with polymers under vacuum (Great Britain Patent No. 1,593,288); impregnation of a porous calcium phosphate body with the melt or solution of prepolymers and solidifying the polymers by further polymerization or curing in the pores or by evaporation of the solvent (U.S. Pat. No. 4,192,021); impregnation of a porous calcium phosphate body with a very reactive monomer like an .alpha.-cyanoacrylate or monomer and catalyst and polymerizing by heating (U.S. Pat. No. 4,192,021); compression molding of an intimately blended, finely powdered mixture of polymer and calcium phosphate (U.S. Pat. No. 4,192,021); and embedding calcium phosphate particles into resins where the calcium phosphate particles have previously been coated with a resin-affinic material to ensure good bonding to the resin, or copolymerizing precoated particles with the resin monomers (German Patent No. 2,620,907).
Calcium phosphate-polymer composite materials can also be used in conjunction with metallic or plastic prosthetics to facilitate adhesion and bone growth around the prosthetic (German Patent No. 2,905,647). The composite can also be applied as a coating, for example, to an anodized titanium/aluminum/vanadium alloy hip prosthetic (Great Britain Patent No. 1,593,288). The essential element in anchoring prosthetic devices appears to be the induction of new bone growth around the device by assuring that contact with the surrounding tissue is through a sheath of, or a surface laden with, bioactive calcium phosphate.
It is desired to have a composite material which is gradually absorbed by the host and is simultaneously replaced with bone tissue without any undesirable side effects such as extensive inflammation or extensive formation of connective tissue. It is also desirable to have a composite material based on calcium phosphates which has improved mechanical properties for use as hard tissue prosthetics over calcium phosphates alone. The prior art teaches composites which are comprised of certain polymers and certain calcium phosphates (U.S. Pat. No. 4,192,021); however, the calcium phosphates in composites of this type are taught as requiring a sintering step.